1. Technical Field
The present invention relates to a projection display device such as a projector.
2. Prior Arts
The projection display device needs to further enhance the contrast of a projected image because the projected image is significantly affected by projection environments.
A single panel projector employing a color switch is disclosed in Japanese Unexamined Patent Publication No. 2001-346219. The projector achieves the enhanced image contrast by taking a procedure of calculating an average luminance of input image signals and controlling the transmittance of the color switch based on the resultant average luminance. This prior-art technique has the following problem. Since the transmittance of the color switch is controlled based on the average luminance of the input image signals, the maximum luminance cannot be attained as the result of the rounding operation. In addition, this prior-art technique is limited to the single panel projector.
On the other hand, a technique of using plural light emitting diodes (LED) as a backlight of a liquid crystal display is disclosed in Japanese Unexamined Patent Publication No. 2006-128125. This prior-art technique offers effective means for enhancing the contrast of image on a direct-view LED display. However, plural light sources are necessary for adapting this technique to the projector, resulting in the increase of cost and size of the display. It is impracticable to apply this technique to the display.